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Old 15-02-2018, 16:09   #1
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Porpoising dinghy

Iíve just got my new dinghy an AB Navigo 12vs. I had a 13í Avon Adventure, but it became too old and frail. Unfortunately I am not happy with the new one. Itís a real step backwards. It has a rock hard ride, is wet and worst of all, it porpoises. Itís even slower despite being much lighter.

I tried raising the engine, a Suzuki 30 to the max and that helped a little. I have power tilt, but the tilt makes it worse. Putting weight and bodies at the bow doesnít help, neither does altering the speed. It will porpoise whatever I do.

I can see now, too late, it has a really flat hull shape, little draft and little deadrise. That possibly explains the hard ride and maybe the porpoising too I wonder.

Any ideas?
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Old 15-02-2018, 16:53   #2
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Re: Porpoising dinghy

Proposing is usually caused by the engine being tilted too high, raising the bow too much, lower the engine and usually the proposing goes away.
Your power tilt, before you get on plane, the engine should be all the way down, then after getting on plane, raise the engine until just before the prop ventilates, the Boat proposes or the bow rides too high, lower the engine before high speed maneuvering like turning around towing skiers or the prop may ventilate in the turn.
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Old 15-02-2018, 16:58   #3
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Re: Porpoising dinghy

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Proposing is usually caused by the engine being tilted too high, raising the bow too much, lower the engine and usually the proposing goes away.
Your power tilt, before you get on plane, the engine should be all the way down, then after getting on plane, raise the engine until just before the prop ventilates, the Boat proposes or the bow rides too high, lower the engine before high speed maneuvering like turning around towing skiers or the prop may ventilate in the turn.
Minimum tilt works best, but it still porpoises badly and alternates between ploughing the nose and slowing down and bouncing up. If I want to ride a mad bucking bronco then I put the tilt on.
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Old 15-02-2018, 17:04   #4
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Re: Porpoising dinghy

By chance do you have the correct engine length? What does that model of dink call for, 20Ē?
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Old 15-02-2018, 17:46   #5
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pirate Re: Porpoising dinghy

According to their spec your 10hp over the recommended engine.. so you'd need the 20" shaft I guess.

Overall length
12' / 3,66m
Overall Beam
6í / 1.82m
Inside Length
8' 9" / 2,67m
Inside Beam
2' 9" / 0,84m
Tube Diameter
18" / 0,46m
Number of Chambers
3+1
Person Capacity
6
Weight
202lbs / 92kg
Recommended HP
20hp / 14,9kw
Maximum HP
30hp / 22,4kw
Shaft length Option 1
15" / 38,1cm
Shaft length Option 2
20" / 50,8cm
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Old 15-02-2018, 18:00   #6
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Re: Porpoising dinghy

This dink comes in a short and long shaft version. I have the long version and the long engine.

I think the fact that the dink can do a short shaft with the normal transom is a sign of its shallow hull. This might be part of the problem.

Engine power isn’t an issue. It porpoises on half or less throttle as certainly as with full power.
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Old 15-02-2018, 18:06   #7
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pirate Re: Porpoising dinghy

Talk to the dealer who sold it to you.. even better take him out in it and let him try.. or have you moved on since then.
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Old 15-02-2018, 18:15   #8
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Re: Porpoising dinghy

Quote:
Originally Posted by poiu View Post

Engine power isnít an issue. It porpoises on half or less throttle as certainly as with full power.

Power maybe not, but what about engine weight?

Is this a tiller steered engine? Are you sitting in the back as well?
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Old 16-02-2018, 03:06   #9
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Re: Porpoising dinghy

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Talk to the dealer who sold it to you.. even better take him out in it and let him try.. or have you moved on since then.
I did. He agrees there is a problem. He will talk to AB for me. No response after 10 days. Iím probably going to be fudged and fuddled I fear.
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Power maybe not, but what about engine weight?

Is this a tiller steered engine? Are you sitting in the back as well?
It has a jockey console, with a wheel. Located centrally with fuel under the wheel. So the weight is already a little forwards compared to usual. The Suzuki motor is 70kg. Super light and outstandingly good by the way.
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Old 16-02-2018, 03:10   #10
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Re: Porpoising dinghy

I note they recommend 20 but say maximum 30. I wonder if it's all just too much power and too heavy. Power tilt doesn't come without a fair weight penalty.

My dinghy is nearly 1 metre longer and it's rated for a max of 30, but that's PVC and it needs the power. I would have thought a 20hp on a glass bottom of that length would be more than enough.

70kg's is not light. My 30hp is 52 kg's.
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Old 16-02-2018, 05:20   #11
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Re: Porpoising dinghy

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I note they recommend 20 but say maximum 30. I wonder if it's all just too much power and too heavy. Power tilt doesn't come without a fair weight penalty.

My dinghy is nearly 1 metre longer and it's rated for a max of 30, but that's PVC and it needs the power. I would have thought a 20hp on a glass bottom of that length would be more than enough.

70kg's is not light. My 30hp is 52 kg's.
A 4 stroke power trim with starter will be normally 80kg plus in 30hp. Relative to other comparable engines it is light. The weight anyway doesn't seem to make much difference. 18kg more than the minimum for an engine is not a big impact compared to passengers. If I have 3 people (250kg) then the problems are still there no matter where I place the weight.

I could make do with 20hp for sure, but I have tried using less power by using less throttle and simulating how it would be with say 15hp and again, it makes little difference.

This boat would be happy with maybe 10 to 15hp with just one person. For 3 or 4 it really needs 30hp.
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Old 16-02-2018, 05:41   #12
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Re: Porpoising dinghy

Look into using Dole Fins, by Davis. Website says from 5hp and up, including outdrives and outboards with powertrim/tilt. Solved the issues when I had an overpowered Caribe RIB.
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Old 16-02-2018, 05:48   #13
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Re: Porpoising dinghy

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Look into using Dole Fins, by Davis. Website says from 5hp and up, including outdrives and outboards with powertrim/tilt. Solved the issues when I had an overpowered Caribe RIB.
I thought those fins were really about getting you on the plane fast?
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Old 16-02-2018, 05:53   #14
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Re: Porpoising dinghy

It does, but it also stabilized the ride and ended the porpoising that occurred. It went from totally unsafe to controllable.
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Old 16-02-2018, 06:09   #15
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Re: Porpoising dinghy

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It does, but it also stabilized the ride and ended the porpoising that occurred. It went from totally unsafe to controllable.
Thanks. I wonder if there are other people with experiences like that?

I did wonder if I could jack the lower bolts back a bit to put more nose down thrust to try to improve things.
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